1. Field of the Invention
This invention relates to an apparatus for reproducing a magneto-optical disk, and more particularly to a magneto-optical disk reproducing apparatus which is adapted to reproduce data from a magneto-optical disk having a recording medium which permits optical recording and erasure of data.
2. Description of the Prior Art
There have recently been proposed a variety of apparatus for recording and/or reproducing a magneto-optical disk which allows data to be rewritten thereon, for example, as disclosed in Laid-open Japanese Patent Application No. 56-19176.
FIG. 1 shows an example of an optical system of the prior art for recording data on and reproducing the same from a magneto-optical disk.
In FIG. 1, reference numeral 1 designates a magneto-optical disk formed of a disk substrate 2 made of glass or synthetic resin such as polycarbonate or the like on which a perpendicular magnetization layer 3 is deposited.
Reference numeral 4 designates a magnet arranged in face of the perpendicular magnetization layer 3 of the disk 1. The magnet 4 is used to record data on the disk 1 and erase data recorded on the same.
When the data recorded on the disk 1 is to be erased, the magnet 4 is rotated to face e.g. the N pole thereof with the surface of the disk 1, and under this condition, the disk 1 is irradiated with an erasing laser light emitted from a semiconductor laser device 5 through a collimating lens 6, polarizer 7, beam splitter 8 and an objective lens 9, whereby the magnetizing direction of the perpendicular magnetization layer 3 is oriented in one direction to erase the data.
For recording data on the disk 1, the magnet 4 is rotated to face e.g. the S pole thereof with the surface of the disk 1 in response to an information signal to be recorded, and the laser device 5 emits a recording laser light corresponding to the data to be recorded with which the disk 1 is irradiated through the collimating lens 6, the polarizer 7, the beam splitter 8 and the objective lens 9. By the above-mentioned operations, the magnetizing direction of the perpendicular magnetization layer 3 of the disk 1 is inverted only when the disk 1 is irradiated with the laser light to thereby effect the data recording.
Incidentally, the example of FIG. 1 needs a mechanism for rotating the magnet 4. It is possible to replace the magnet 4 with a coil which inverts the polarity of its magnetic field in response to information signals, wherein the polarity of the magnetic field can be inverted only by supplying the opposite-phase current to the coil, with the result that the mechanism can be simplified.
For reproducing the data thus recorded on the disk 1, the disk 1 is irradiated with the reproducing laser light from the laser device 5 through the collimating lens 6, the polarizer 7, the beam splitter 8 and the objective lens 9 in a manner that the laser light is focused on the perpendicular magnetization layer 3. The laser light beam reflected from the disk 1 is supplied through the objective lens 9 to the beam splitter 8 which reflects the laser light beam in the perpendicular direction to the incident laser light beam so as to supply the same to a polarized beam splitter 12 through a half wave (.mu./2) plate 10 and a condenser lens 11. A polarized light component of a first polarization plane (for example, the same as that of the laser light irradiated on the disk 1) of the light beam which passes through the polarized beam splitter 12 is incident on a photo-diode 13A whose output S.sub.A is supplied through an amplifier 14A to a differential amplifier 15 at one of its input terminals. A polarized light component of a second polarization plane, perpendicular to the first polarization plane of the laser beam is incident on a photo-diode 13B whose output S.sub.B is supplied through an amplifer 14B to the differential amplifier 15 at the other input terminal thereof.
The light beam reflected on the disk 1 is such that its polarization plane is rotated by the magnetic Kerr effect, dependent on the magnetizing direction of the perpendicular magnetization layer 3. For example, when the magnetizing direction of the perpendicular magnetization layer 3 is oriented to a first direction, the polarization plane of the reflected light beam is rotated by .theta..sub.k. On the other hand, when the magnetizing direction is a second direction opposite to the first direction, the polarization plane is rotated by -.theta..sub.k. Therefore, when the magnetizing direction of the perpendicular magnetization layer 3 is in the first direction, the polarized light component of the first polarization plane of the light beam which is led to the photo-diode 13A through the polarized beam splitter 12 is decreased or increased, while the polarized light component of the second polarization plane of the light beam which is led to the photo-diode 13B after being reflected by the polarized beam splitter 12 is increased or decreased. On the other hand, the magnetizing direction of the perpendicular magnetization layer 3 is in the second direction, the opposite states to the above states will occur. Thus, the differential amplifier 15 delivers to an output terminal 16 a signal So corresponding to the data recorded on the disk 1.
With the apparatus as shown in FIG. 1, a carrier to noise (C/N) ratio upon reproduction is substantially proportional to the product of the square root of an amount Io of the light incident on the disk 1 and the rotating angle .theta..sub.k, as expressed by the following equation: ##EQU1##
Therefore, it can be thought from the equation (1) to increase the incident light amount Io by elevating the power P of the laser device 5 in order to increase the C/N ratio. However, only increasing the incident light amount Io will result in that the temperature of the perpendicular magnetization layer 3 is locally increased, leading to .. decrease in the rotating angle .theta..sub.k and consequently suppressing the C/N ratio from being increased. Specifically explaining with reference to FIG. 2, when the power P of the laser device 5 is elevated upon reproduction, the C/N ratio is initially increased. However, if the power P becomes above a certain constant value or above 1 mW in the example of FIG. 2, the C/N ratio is decreased to the contrary. This tendency becomes stronger as a linear velocity of the disk upon reproduction is decreased from V.sub.1 to V.sub.2, V.sub.3 . . . .
The assignee of the present application has previously proposed an apparatus for reproducing a magneto-optical disk which is capable of increasing the incident light amount Io without increasing the temperature on the perpendicular magnetization layer 3 to thereby increase the C/N ratio (refer to Japanese Patent Application No. 62-3398).
This previously proposed apparatus is adapted to intermittently irradiate the laser light on the disk 1 in a sampling period, that is, the disk 1 is not continuously irradiated with the laser light, whereby the increase in the incident light amount Io does not cause the increase of temperature on the perpendicular magnetization layer 3.
Let it now be assumed that data is being recorded on the disk 1 as shown in FIG. 3A. The laser device 5 is controlled to be intermittently turned on at a sampling period t as shown in FIG. 3B to emit the laser light. The period t.sub.ON during which the laser light is emitted is assumed to be 1/5-1/20 the period t.
When the laser light is emitted from the laser device 5, a polarized light, the polarizing plane of which is rotated corresponding to the data recorded on the disk 1, is obtained from the disk 1, so that the output signals S.sub.A and S.sub.B from the respective photo-diodes 13A and 13B are as shown in FIGS. 3C and 3D, and consequently the signal So corresponding to the data, as shown in FIG. 3E is derived at the output terminal 16 from the differential amplifier 15.
As described above, it is possible to largely reduce a mean value of the incident light amount of the laser light by intermittently illuminating the laser light to the disk 1, avoid the increase of temperature on the perpendicular magnetization layer 3, and thereby increase the C/N ratio.
The relationship among the incident light amount Io, carrier C, cophasal (same phase) noise N.sub.SA and shot noise N.sub.SH, thermal noise N.sub.TH is as shown in FIG. 4A. To be specific, the carrier C is proportional to the incident light amount Io. The cophasal noise N.sub.SA, which results from an unbalanced amount of the laser light incident on the photo-diodes 13A and 13B due to the difference in the rotating angle of the polarized plane caused by the .lambda./2 plate 10 or the like, is proportional to the incident light amount Io. The shot noise N.sub.SH, which occurs in the procedure of photo-electric conversion in the photo-diodes 13A and 13B, is proportional to the square root of the incident light amount Io.
From the relationships shown in FIG. 4A, the C/N ratio can be illustrated as shown by a solid line in FIG. 4B. However, the C/N ratio expressed by the solid line is obtained without considering decrease in the rotating angle .theta..sub.k due to the elevation of the temperature caused by the increase in the incident light amount Io. A practical C/N ratio is such as shown by a broken line in FIG. 4B. It is therefore possible to make a curve indicative of the practical C/N ratio close to the solid line in FIG. 4B by intermittently irradiating the laser light on the disk 1, as described above.
Since the shot noise N.sub.SH is proportional to the square root of the incident light amount Io, as described above, increase in the C/N ratio by increasing the incident light amount Io is merely increased in proportion to the square root of the incident light amount Io also when the disk 1 is intermittently irradiated with the laser light.
However, the shot noise N.sub.SH is a random noise so that it cannot be removed by the differential amplifier 15 without any modification.